The disclosed embodiments of the invention pertain to a miniature reference electrode for use in the chemical sensor or biosensor based on the electrochemical principle in many electrochemical experiments, and in particular, to a miniature silver/silver chloride reference electrode with high stability and general applicability as a reference electrode.
A reference electrode is an electrode used as a reference in a potential setting or potential measurement. It is an important structural element indispensable not only in the basic studies of electrochemistry, but also in the application research of chemical sensors. For example, in cyclic voltammogram and other experiments with the electrode potential included as an information, there is a demand on a reference electrode that can provide a stable reference potential. Also, for the chemical sensors and biosensors, in particular, for the potentiometric sensors for potential measurement, the potential of the indicator electrode that measures the chemical substance is measured using the reference electrode as a standard, and the concentration of the chemical substance is measured with the potential as an index. Consequently, if a highly stable and reliable reference electrode is not used, the deviation in the potential leads directly to a measurement error.
Consequently, the reference electrode is a basic and important element for the electrochemical sensor. At present, the silver/silver chloride electrode is most widely used as the reference electrode. The structure of the silver/silver chloride electrode is shown in FIG. 1. In container (4) made of glass, rod-shaped xe2x80x9csilver/silver chloridexe2x80x9d (1) is set. In the end portion of glass container (4), a liquid junction (3) made of a ceramic plug or the like is formed. The interior of the container (4) is filled with saturated KCl solution or saturated NaCl solution.
In recent years, there has been progress achieved in miniaturizing the chemical sensors and biosensors. Consequently, there has been a demand for development of a miniature reference electrode that can provide a stable potential. The miniature silver/silver chloride electrode conventionally used to meet this demand is shown in FIG. 2. On substrate (10), base metal layer (9), silver layer (6) and silver chloride layer (7) are sequentially laminated, and xe2x80x9csilver/silver chloridexe2x80x9d is only in a simple thin film form.
For the thin-film silver/silver chloride electrode (FIG. 2) as the miniature reference electrode, the xe2x80x9csilver/silver chloridexe2x80x9d portion (the portion of silver layer (6) and silver chloride layer (7)) is prepared as follows. For example, silver layer (6) is formed on a layer of chromium or titanium as the adhesion layer (9). Its surface is partially converted to silver chloride. In certain cases, in order to further improve the adhesion property of silver, an intermediate layer made of gold or platinum or the like is included between adhesion layers (9) and silver [layer] (6) in consideration of the structure or process of the sensor. (Also, this structural body is usually formed by patterning with the aid of photolithography).
The thin-film silver/silver chloride electrode (FIG. 2) is usually used by placing it in direct contact with the sample solution. However, for the silver/silver chloride reference electrode, the electrode potential varies as a function of the active amount of chlorine ions. Consequently, in order to perform an appropriate measurement, it is necessary to use it in a solution having a predetermined active amount of chlorine ions. Consequently, when it is used in combination with a certain micro indicator electrode, a solution with a predetermined active amount of chlorine ions is prepared separately and brought into contact with the xe2x80x9csilver/silver chloridexe2x80x9d portion, and an appropriate amount of sample solution is added to perform the measurement.
However, in this prior method, each time it is used, one has to prepare a solution with a predetermined active amount of chlorine ions. Also, when the sample is injected, the active amount of chlorine ions varies a little, leading to unwanted variations in the potential. Also, in the case when a sample is measured without dilution as the level best for the practical application, it is very difficult to measure the active amount of chlorine ions beforehand, and the measurement value itself is unreliable.
Consequently, when measurement with a high reliability is to be carried out, as shown in FIG. 1, one must make the xe2x80x9csilver/silver chloridexe2x80x9d portion in contact with a saturated solution of KCl and make the connection to the outside through a liquid junction.
However, for the structural body of the thin-film silver/silver chloride reference electrode shown in FIG. 2, as it is a thin film, the silver chloride layer (7) is very thin. Consequently, the durability is low, and it is damaged in a time as short as a few minutes in a saturated solution of KCl. Consequently, there is yet no commercially available product using this structure directly (that is, the structure in which solution of KCl or NaCl in a saturated or nearly saturated concentration is filled as the internal electrolyte, and connection to the outside is performed by means of a liquid junction).
The disclosed embodiments of the present invention provide a miniature reference electrode with improved stability and durability. In particular, the embodiments of the invention provides a miniature silver/silver chloride reference electrode with high stability and general applicability as a reference electrode.
A xe2x80x9csilver/silver chloridexe2x80x9d structural body is shown in FIG. 3. It differs from a conventional product in that the surface of silver thin film (6) is covered with a water-repellent polymer layer (8) (for ease of reference, the same part numbers as in FIG. 2 are adopted here for similar elements), and silver chloride layer (7) is formed from the end portion of silver thin film (6) instead of from the surface of the thin film. It was found that the lifetime of the thin-film xe2x80x9csilver/silver chloridexe2x80x9d structural body in the saturated aqueous solution of KCl or AgCl was prolonged by nearly 300-fold. Thus, the thin-film xe2x80x9csilver/silver chloridexe2x80x9d structural body is used as the reference electrode having the liquid junction as explained above.
The thin-film xe2x80x9csilver/silver chloridexe2x80x9d structural body can be manufactured as a reference electrode having a liquid junction (15) (FIG. 4). In this case, the container portion (16) for storing the electrolyte (saturated aqueous solution of KCl or AgCl) and liquid junction (15) arc formed on a silicon substrate (14), and it is bonded to a glass substrate (17). The thin-film xe2x80x9csilver/silver chloridexe2x80x9d structure (18) is thus formed.
For the reference electrode with a liquid junction, when it is in use, an electrolyte is injected from outside (11) of the silicon substrate through an electrolyte feeding port (13), the end portion of liquid junction (12) is recessed into a predetermined solution, and the portion of pad (19) is connected to a terminal for measurement. The reference electrode having the liquid junction shown in FIG. 4 can display the desired stable potential.
However, for the miniature reference electrode of the aforementioned prior art, there were several problems. The problems of the miniature reference electrode of the aforementioned prior art and the methods for solving them, more specifically, the problems that take place in company with setting of the aforementioned liquid junction and the methods for solving them, are as follows.
The first problem is as follows: When the electrolyte (KCl) is introduced as an aqueous solution, due to flow-out of KCl inside it and flow-in of water outside it, the concentration of KCl of the internal solution significantly decreases to a measurement error, and the lifetime becomes as short as a few hours. On the other hand, when more KCl is stored in a deposited state, this problem is expected to be improved, yet in the system in which the electrolyte solution is introduced later, such as the reference electrode shown in FIG. 4, as the size becomes smaller, it becomes extremely difficult. This problem can be solved by forming the layer of KCl or NaCl as the electrolyte in powder form without dissolution. More specifically, this problem is solved by the following method: KCl or NaCl powder as the electrolyte cannot be directly used to form an electrolyte pattern, it is dispersed in a paste prepared using an organic solvent, and the paste is applied by screen printing on the necessary sites, followed by dissipation of the solvent to form the desired electrolyte layer.
The next problem is as follows: For the liquid junction, if a structure in which a fine polymer layer is filled and flow-out of KCl is inhibited is not adopted, the internal KCl will be soon lost. This problem can be solved by the following method: A hydrophilic polymer film is used for the liquid junction, and, in order to suppress the flow-out rate of KCl, KCl is made to flow out in the direction perpendicular to the film""s cross-section. (If the internal liquid flows out in the direction perpendicular to the film surface, the flow-out rate of KCl becomes higher).
The third problem is as follows: Degradation of ""silver/silver chloride"" starts at the time when contact is made with the electrolyte. Consequently, it is preferred that the electrolyte solution be introduced right before use. However, for the reference electrode shown in FIG. 4, it is necessary to introduce the electrolyte solution by means of a degassing operation and centrifuge. Consequently, it is usually very difficult for conventional users. This problem can be solved by the following method: Since degradation of the thin-film ""silver/silver chloride"" starts at the time when contact is made with the saturated aqueous solution of KCl, activation is carried out right before use. When this is implemented, the saturated solution of KCl or AgCl is made to pass through a passivation layer for injection by means of a microsyringe.
The miniature reference electrode of the disclosed embodiments herein has the following features.
The miniature reference electrode contains a thin-film backbone metal pattern set on an electrode substrate, a thin-film electrode metal pattern set on said thin-film backbone metal pattern and is in electrical contact with a portion of said thin-film backbone metal pattern, a metal salt region which is set on a portion of said thin-film electrode metal pattern and is made of the salt of the structural metal of said thin-film electrode metal pattern, a thin-film electrolyte layer which is a thin-film electrolyte layer impregnated with the saturated solution of electrolyte shared with the anionic portion of said metal salt region and which has a portion in contact with said thin-film metal salt layer via a hydrophobic thin-film insulating layer, and a thin-film liquid junction pattern which is in contact with a portion of said thin-film electrolyte layer and is prepared by impregnation with an electrolyte solution containing the electrolyte of said thin-film electrolyte layer; while the overall miniature reference electrode is covered with a protective film, at least a portion of said thin-film backbone metal pattern is exposed as the electrode terminal portion, and a portion of said thin-film liquid junction pattern is exposed as a contact portion with the external liquid.
The miniature reference electrode described above has a thin-film electrolyte layer with the electrolyte carried in the form of solid particles.
The miniature reference electrode in one embodiment, is formed so that the metal of said thin-film electrode metal pattern is silver, the metal salt of said metal salt region is silver chloride, and the electrolyte of said thin-film electrolyte layer is potassium chloride. The miniature reference electrode may have the metal of the thin-film backbone metal pattern formed of gold.
The miniature reference electrode described above may have a plurality of holes formed on the thin-film electrode metal pattern, and, as the hydrophobic thin-film insulating layer is bonded with said electrode substrate through the holes, the adhesion of the thin-film electrode metal pattern on the electrode substrate is increased.
A one-touch potentiometric sensor may be formed using the miniature reference electrode described above and a sensor based on the potentiometric measurement principle that are integrated in a single step.
A one-touch electrochemical measurement system is provide, with a function electrode, a counter electrode, and the miniature reference electrode described above integrated on a single chip.
The miniature reference electrode has the electrode substrate, and an auxiliary electrode pattern is set on the electrode substrate via another metal pattern and a thin-film insulating layer.
A method for prolonging the lifetime of the miniature reference electrode made of plural thin-film layers is also disclosed. The method includes, in the miniature reference electrode made of plural thin-film layers, forming a portion of a thin-film electrode metal pattern to have a metal salt region made of the salt of the structural metal of the thin-film electrode metal pattern, and the metal salt region is exposed on the surface of the thin-film electrode metal pattern; a portion of said metal salt region is a thin-film electrolyte layer impregnated with the saturated solution of an electrolyte shared with the anionic portion of said metal salt region, and it is in contact with a portion of the thin-film electrolyte layer which has said electrolyte carried in the form of solid particles; and a portion of said thin-film electrolyte layer is liquid junctioned to the outside.
In accordance with another embodiment of the invention, a miniature silver/silver chloride reference electrode is provided which includes: on a substrate, a pattern made of a noble metal and acting as a backbone; a silver pattern is formed such that it makes contact with a portion of the noble metal pattern; an insulating layer that does not allow permeation of aqueous solution is covered on the entire surface of the silver pattern and the portion other than the pad portion; on the silver pattern, an opening is formed in said insulating layer; a silver chloride layer is formed from the slit through the interior of the silver pattern; also, a hydrophilic porous layer as the liquid junction is formed; a hydrophilic porous layer containing particles of chlorine-containing salt and electrolyte solution is formed such that the slit and the liquid junction pattern are contained; and a protective film that does not allow permeation of the aqueous solution is formed such that a portion of the liquid junction and the pad portion is left, and at least the entire region of the hydrophilic porous layer that contains the particles of chlorine-containing salt and electrolyte solution, is covered.